Thermo-responsive hydrogel has rarely been applied in acoustic applications except for using a phononic crystal-based acoustic lens with linear transmission within the first phononic transmission bandgap using a homogenized medium concept. However, the higher transmission bands can offer metamaterial behavior with a negative index or anomalous dispersion characteristics. In the present study, the thermally responsive hydrogel-infilled phononic crystal lens was designed for the second and third transmission bands and served as a meta-material lens. The band structure and equifrequency contours of the proposed phononic crystal were calculated based on the frequency- and temperature-dependent speed of sound. A specific operating frequency range within the medium’s critical phase transition temperature induces a strong temperature-sensitive equifrequency topology that enables a negative to positive index of refraction shift. The acoustic focusing behaviors of the designed meta-material lens were verified with numerical simulation and experimental characterizations. The temperature-tunable focusing behaviors were applied to perform monostatic detection showing subwavelength resolution (~ 0.84 \(\lambda\)) with temperature-tunable detection distances.